Many types of telephone sets are currently available in the market; however, their ringing circuits are nearly the same. When a conventional telephone is ringing, its sound is very loud and noisy. When several sets of conventional telephones are closely installed, it is hard to distinguish which one is ringing as each one rings the same identical way. Furthermore, the conventional telephone sets produce undesirable sounds because of the transient oscillating current generated by the consecutive opening and closing of the contacts in the telephone sets when dialing the telephone. Another disadvantage of conventional telephone sets is that the modification of the ringing circuit in order to change the level or tone of the ringing sounds is not easy; for example, the circuit disclosed in U.S. Pat. No. 3,135,896 entitled "Narrow Band Sensing Circuit" operates only with a ringing signal of 25 cycles/sec.